Updated Nov. 5, 7:23 a.m. | Republicans swept the Senate races Tuesday night, and come January, they will control the chamber for the first time in eight years.

Democratic incumbents fell right and left, even in seats that they had originally been favored to win. President Barack Obama's poor approval rating — 42 percent in the last nationwide Gallup poll — dragged down candidates across the country in the face of a Republican wave.

Roll Call's final ranking of the most vulnerable senators doesn't vary much from previous versions — the result of an unfavorable national climate for Democrats that has failed to improve.

On the eve of the midterm elections, Senate Democrats are staring down a hole dug by President Barack Obama’s disapproval ratings and an unforgiving map packed with red states. Retirements by a quartet of senators in Republican-leaning or swing states didn’t help, but the seats of at least four incumbents seeking re-election aren’t on much stronger ground.

The winner of the race between Roberts, left, and Orman, right, will play a major role in deciding the Senate majority. (Tom Williams/CQ Roll Call)

So much for a predictable midterm cycle. The past month has left multiple possible outcomes for control of the Senate.

Republican groups are barraging Kansas with resources and advertising to save a three-term incumbent being challenged by an independent in a solidly GOP state. Democrats, lacking much hope for months of holding an open seat in South Dakota, are all of a sudden dropping $1 million in advertising there — and being matched by Republicans — in a last-second Hail Mary that could possibly save its majority.

The National Republican Senatorial Committee launched its first TV ad today in the newest hot race on the competitive Senate playing field, one day after national Democrats released an ad of their own.

The spot, part of the committee's recently-announced $1 million ad buy in the state, takes aim at Democrat Rick Weiland and independent Larry Pressler — the two greatest challengers to Republican former Gov. Mike Rounds.

Roberts, left, greets Moran, the NRSC chairman, at an event in their home state of Kansas. (Tom Williams/CQ Roll Call Photo)

Updated, 9:04 a.m. | TOPEKA, Kan. — With less than four weeks until Election Day, the National Republican Senatorial Committee's independent expenditure arm is shifting resources to increase its investment in six states, including South Dakota and Georgia.

The NRSC has moved $1 million to South Dakota, plus another $1.45 million to Georgia.In South Dakota, the Democratic Senatorial Campaign Committee made a $1 million television ad buy this week, on the heels of tightening poll numbers that showed its candidate, Rick Weiland, gaining ground. In Georgia, a new poll suggests a runoff is likely. The NRSC also is upping its investment in four other states: Alaska, Colorado, Iowa and New Hampshire. Those four states represent an expansion of the map for the GOP into states where Democrats were favored to win earlier in the cycle.

OVERLAND PARK, Kan. — The power in the Senate could increasingly flow not to Harry Reid or Mitch McConnell, but to a few independents who could hold the keys to the majority — and they know it.

The two unexpected GOP trouble spots in the Midwest feature independent candidates who are making noise about not joining either side in a divided Senate. In Kansas it's Greg Orman, who is challenging long-time GOP incumbent Pat Roberts. Republicans are extremely dubious of Orman, pointing to campaign dollars he's given to top Democrats, although Orman is fond of pointing to contributions to Republicans as well.

The Democratic Senatorial Campaign Committee will run $1 million in advertising in the South Dakota Senate race — a welcome, but not surprising development for the campaign of Democrat Rick Weiland.

A Weiland campaign senior adviser and veteran Democratic operative said he expected the move, given Senate Majority Leader Harry Reid's interest in remaining in charge. Steve Jarding, a South Dakota native who helped elect future Senate Majority Leader Tom Daschle, D-S.D., in 1986 and worked for four years at the DSCC, told CQ Roll Call moments after Bloomberg Politics broke the story on Wednesday that he had a feeling the national party would eventually invest there.